Photovoltaic power generation is a clean method of generating electric power using inexhaustible light energy without discharging toxic substances. A solar cell is used for the photovoltaic power generation, which is a photoelectric converter that generates electric power by converting light energy from the sun into electric energy.
Conventionally, an electrode on the back of a light receiving surface of a generally produced solar cell is formed by screen-printing silver paste and aluminum paste on the back surface of a silicon substrate, then drying and firing the pastes. The aluminum formed substantially all over the back surface of the silicon substrate serves as a positive electrode. However, in the process of producing a solar cell module, a lead tab for extracting electric power cannot be soldered directly to the aluminum electrode formed of aluminum. Therefore, a silver electrode is formed, as an electrode for extracting electric power, in such a manner as to partially overlap the aluminum electrode on the back surface of the silicon substrate (see Patent Documents 1 and 2).
As just described, on the back surface of the substrate of the solar cell, an aluminum electrode for higher electric power output and a silver electrode for extracting the electric power are partially overlapped. In the area where the aluminum electrode and the silver electrode are overlapped, three metals of silicon in the silicon substrate, aluminum in the aluminum electrode, and silver in the silver electrode are partially alloyed.    Patent Document 1: Japanese Patent Publication No. 2003-273378    Patent Document 2: Japanese Patent Publication No. HEI 10-335267